Joint replacement surgery is quite common and it enables many individuals to function normally when they otherwise would not be possible to do so. Typically, an artificial joint includes metallic, ceramic and/or plastic components that are fixed to existing bone. One of the more common joints that undergoes replacement surgery is the knee. Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint.
Artificial knee joints consist of essentially four components. The first component is a metallic tibia implant. The second component is a metallic femoral implant. The third component is a high-density polyethylene insert positioned between the two metal components. The fourth component, not part of the present invention, is a polyethylene patellar component. Together they form a joint that can provide a total replacement for a diseased native knee joint.
Traditionally the joint surfaces associated with the implant components are approximated by toroidal or donut shaped surfaces on both the insert and the condylar surfaces of the femoral component which transfer load from the femur to the tibia through the polyethylene insert. The total joint, once implanted, is stabilized and controlled in part by these surfaces and in part by the soft tissues surrounding and encapsulating the knee.
TKA devices can fail for reasons such as aseptic loosening, instability, or infection. Failure usually requires revision surgery. Revision implants have been developed that include a post on the polyethylene tibial component that articulates within a recess (intercondylar box) in the femoral component. The objective of this so called constrained condylar knee (CCK) implant is to rely on contact between the box and the post within the joint itself to restrain and limit rotation of the knee (varus/valgus rotations). This constraint is also beneficial in primary TKA if the soft tissues cannot be balanced to achieve an adequately stabilized and controlled joint.